The present invention relates to the identification of soil contaminants in subsurface environments. More specifically, but without limitation thereto, the present invention relates to a probe for laser induced breakdown spectroscopy of subsurface soil.
Increasing concern with soil and groundwater contamination and governmentally mandated requirements to clean up hazardous waste sites have created a need for cost effective systems and methods for determining the characterization of subsurface environments. In response to such needs, soil penetrating probes have been developed generally comprising a tube with a tapered tip which is forced into the ground. Instrumentation inside the tube detects various properties of the surrounding geological environment.
Laser Induced Breakdown Spectroscopy (LIBS) is a method for detecting the presence of various elements in a sample by directing a high power emission from a laser onto the sample to form a plasma. The plasma is then analyzed spectroscopically to determine the composition of the sample. The LIBS technique offers promise as a method suitable for use with a soil penetrating probe to detect heavy metal contamination in soil, because it is highly sensitive and requires no sample preparation. Usually the LIBS technique involves delivery of the laser energy to the sample through air due to the high power densities required to ionize the sample. Recently, fiber optic cable has been used successfully in LIBS measurements, allowing measurement of samples that are located at a considerable distance away from the excitation laser and analyzing equipment.
The use of cone penetrometry for elemental and molecular subsurface soil analysis using optical sensors has provided a lower cost alternative to traditional drilling and sampling methods and has been the subject of several recent patents.
U.S. Pat. No. 5,128,882 by Cooper et al. issued on Jul. 7, 1992 discloses a fiber optic cone penetrometer probe to irradiate the soil with UV or visible light to generate a fluorescence, reflection, or absorption spectrum of soil contaminants. The fluorescence spectroscopy described in this patent generates information for classifying certain molecular species, but does not form a plasma and is generally insensitive to atomic species, which are important to the identification of metal contamination.
U.S. Pat. No. 5,316,950 by Apitz et al. issued on May 31, 1994 discloses a method for interpreting fluorescence spectra from a cone penetrometer of the type described by Cooper using strain gauge data to compensate fluorescence measurements for variations in soil matrix.
U.S. Pat. No. 5,379,103 by Zigler issued on Jan. 3, 1995 discloses a dual mode probe using separate optical fibers to conduct excitation and response signals. In one mode the probe uses high peak power focused laser radiation to initiate a laser induced spark in a soil or groundwater sample. The optical emission from the sample is spectrum analyzed to identify and quantify elemental (atomic) species. In the other mode, relatively low peak power laser radiation is used to generate fluorescence to yield information from the irradiated sample about molecular species. Using separate optical fibers for the excitation and response signals becomes impractical as shorter focal lengths are used to increase the power density of the excitation signal, however, because the optical alignment of the fibers becomes more difficult. Another disadvantage is that the probe must be lowered down a well, requiring that a well be drilled first.
Although these recent developments represent considerable progress, much work remains to be done to solve problems such as delivering a high power pulse to a sample through an optical fiber without reducing the optical conductivity of the optical fiber, aligning and focusing the optics within the probe, and decoupling the excitation beam from the response signal. Therefore, there is a continued need for a cone penetrometer that can deliver a high power pulse of energy to a remotely located sample through an optical fiber and to decouple the response from the excitation pulse.